The present invention generally relates to thermal ink jet printing systems, and more particularly to a method and apparatus for cooling the print heads of thermal ink jet systems during operation.
The development of new and improved printing systems has created a corresponding demand for high-efficiency ink cartridges. High efficiency cartridges must be capable of delivering ink in a rapid and continuous manner with a substantial degree of print resolution. This is especially true with respect to thermal ink jet cartridges which operate at high speeds using a jetting resistor assembly. An exemplary thermal ink jet cartridge of this type is illustrated in U.S. Pat. No. 4,500,895.
Thermal ink jet systems typically use a glass or ceramic substrate having a plurality of thin-film jetting resistors attached to the substrate. Also secured to the substrate is an orifice plate made of glass, ceramic, metal, or the like having a plurality of drop expulsion holes therethrough. Each one of the drop expulsion holes is associated with at least one of the thin-film jetting resistors. When the resistors are energized, they correspondingly increase in temperature. As a result of this temperature increase, ink stored within the cartridge is thermally excited and pushed outwardly through the drop expulsion holes in the orifice plate. Thereafter, the ink is ejected from the system. This process is more completely described in the Hewlett-Packard Journal, May 1985, Vol. 36, No. 5.
Thermal ink jet systems of the type described above operate in an efficient manner. However, when operating at high speeds, the resistor assembly and orifice plate can become excessively hot, causing a degradation in print resolution and quality. Specifically, the increase in heat causes larger drops of ink to be expelled from the cartridge which adversely affects print resolution. The increased temperature also causes the viscosity of the ink to decrease. Again, this causes larger drops of ink to be expelled from the cartridge. In order to cool the internal components of the cartridge, one technique has involved the attachment of a metal heat sink unit (e.g. a manifold) adjacent the resistor assembly in the cartridge. However, this method has proven to be impractical from a technical and economic standpoint.
Accordingly, a need remains for a thermal ink jet system having means therein for efficiently cooling the system so that excessive heat generation and print deterioration may be prevented. The present invention satisfies this need as described below.